Every year large numbers of pneumatic tires, in particular automobile tires, are discarded. For example, it was reported in Rock Products, October 1980, that over 200 million automobile tires were being discarded every year in the United States, and that some 300,000 tons of old tires were being generated annually in what was then West Germany. A large proportion of waste tires are simply buried in landfill sites and there have been various proposals for using comminuted waste tires, e.g. in road-building asphalts; nonetheless, the environmentally acceptable disposal of waste tires still presents a considerable challenge.
It has been recognised that the calorific value of waste tires, at 27,000 kJ/kg, is comparable to that of coal and it is known to use chipped or otherwise comminuted tires as an auxiliary fuel in cement kilns. However, when account is taken of the capital investment needed for machines for chipping the tires and the energy costs in operating such machines, the cost benefits obtained by replacing coal or other fossil fuel with tire chips are often not substantial. Furthermore, if the tire chips are to be fed into a precalciner or preheater system, it is necessary carefully to control the velocity of the gas in that system to ensure adequate combustion of the chips before they are swept into the next stage of the preheater.
There have also been proposals for feeding whole tires directly into a rotary cement kiln. For example, U.S. Pat. No. 4,551,051 discloses apparatus by means of which whole tires may be delivered through the mineral-inlet end of a rotary cement kiln to a selected region of the kiln. In U.S. Pat. No. 5,078,594 an apparatus is described for charging whole tires through a port in the wall of a rotating cement kiln into a heated zone of the kiln.
However, the delivery of whole tires directly into a rotary cement kiln requires careful control to ensure that the tires are fully burnt within the kiln, that there is no localised reduction, which can lead to the build-up of solids and blockages in the lower stages of the preheater system, and that the solid residue is uniformly incorporated into the cement clinker.
DE-A-3,326,375 discloses a method for the production of cement clinker in a rotary kiln, in which the raw material is preheated in a multi-stage cyclone preheater and is then calcined using different fuel components in at least two different places in the preheater before entry into the rotary kiln. This method is characterised in that the raw material is preheated and calcined in two separate streams in separate strings of the preheater; in that a coarse particulate solid fuel component (for example used tires or lump coal) is supplied to the first preheater string at a point located close to the rotary kiln such that this fuel component arrives in the rotary kiln before complete combustion; and in that another fuel component, preferably a liquid, gaseous or fine particulate solid fuel component, is introduced into the second string of the preheater such that this fuel component is thoroughly mixed with the material to be calcined before complete combustion. It is suggested in this German patent application that this method permits the use of quite different types of fuel without the formation of coatings or other operational problems in the preheater; it is suggested that this is due to the fact that the coarse particulate fuel component is practically completely gasified in the rotary kiln (page 5, lines 1-6, and page 5, line 25 to page 6, line 1). In the detailed description of an illustrated embodiment, the point at which the coarse particulate solid fuel component is supplied to the first preheater string is selected such that the coarse particulate fuel will fall more or less freely through the inlet housing associated with the rotary kiln and thence into the rotary kiln itself, where it is gasified (page 11, lines 1-9). There is no disclosure or suggestion that the coarse particulate fuel is maintained in contact with the hot gas stream in the preheater; and it would appear that in essence the coarse particulate fuel is essentially subjected to pyrolysis and gasification within the rotary kiln, the resultant gas then being available for combustion in the preheater system.
U.S. Pat. No. 4,295,823 and U.S. Pat. No. 4,627,877 describe, and respectively claim, an apparatus and a method for continuously producing a cement clinker in which a combustible waste material is employed as a heat source. According to the disclosure in these U.S. patents, a cement raw material is fed into a preheating or calcining chamber, from which the resultant preheated or calcined material is forwarded into a heating region within a rotary kiln in order to convert the material into a cement clinker. The cement clinker is then moved into a cooling chamber in which its temperature is decreased by means of cooling air. Exhaust gas from the heating region in the rotary kiln is passed into the preheating or calcining chamber. The combustible waste material is fed into a heat-decomposition chamber into which exhaust gas from the heating region of the rotary kiln is also introduced and in which the combustible waste material is thermally decomposed in order to generate a combustible gas. This combustible gas is then conveyed to, and burnt within, the preheating or calcining chamber to preheat or calcine the cement material. Accordingly, the said waste material undergoes a pyrolysis and gasification process rather than combustion as such. Any solid residue from the thermal decomposition of the combustible waste material is allowed to pass into the heating region within the rotary kiln. Various combustible waste materials are mentioned, including tire wastes, rubber wastes, oil wastes, oil-containing sludges, asphalt wastes, pitch wastes, and organic compound wastes.
There remains a need for an improved method whereby whole tires can be disposed of in a cement-manufacturing process or the like.